Damper assemblies can be divided into two categories: friction dampers having solid elements and hydraulic dampers having fluid elements. Hydraulic dampers are used in the suspension system of a vehicle. Wherein, a piston subassembly moves in a fluid chamber in an upward movement, known as a rebound stroke, and a downward movement, known as a compression stroke. The rate of hydraulic fluid flow allowed through the piston subassembly determines the pressure drop and a damping force. A high damping force results in a stiff suspension and a low damping force results in a soft suspension. A stiff suspension results in a more active performance capability, whereas a soft suspension results in a more comfortable ride experience.
The main shortcoming of conventional damper assemblies is the compromise between stiff suspension and soft suspension. Wherein, stiff suspension includes high damping and active performance and soft suspension includes low damping and a more comfortable ride. Stiff suspension results in active handling, but negatively impacts noise and isolation. Soft suspension improves isolation, but adversely affects active handling.
The piston subassembly in a conventional fluid damper assembly includes a plurality of disks to control fluid flow through the piston subassembly by the force required to open a plurality of passages by flexing the disks. The disks and passages are designed and tuned for an optimum compromise between the stiff suspension and the soft suspension. The changing of damper tuning from a stiff suspension setting to a soft suspension setting requires disassembly of the fluid damper assembly and modifying the force necessary to flex the disks (i.e. stiffness of the disks).
In order to rapidly adjust between stiff suspension and soft suspension without disassembly, an actuator has been incorporated to adjust damping force on the disks from the passages to allow the disks to flex in response to hydraulic fluid pressure and open the passages, thus to adjust and soften the suspension.
Such a fluid damper assembly is disclosed in the U.S. Pat. No. 5,054,809 to Yamaoka et al. which comprises of a housing that has a wall extending annularly about and along a center axis to define a fluid chamber for containing a hydraulic fluid. A piston subassembly defines an upper surface and a lower surface defining a periphery extending axially therebetween and slidable along the wall of the housing between a rebound stroke and a compression stroke. A plurality of inflow passages allow the hydraulic fluid to flow through the piston subassembly during the rebound stroke and at least one rebound disk is disposed on the lower surface of the piston subassembly for restricting flow of the hydraulic fluid through the inflow passages. The rebound disk is seated on a retainer and a spring biases the retainer toward the rebound disk. A fastener abuts the spring for adjustably preloading the spring axially against the retainer to bias the retainer against the rebound disk. The rod includes a piezoelectric actuator for exerting an axial force directly on the rebound disk in directions opposite the biasing force of the spring to adjust between stiff suspension and soft suspension.